Optical pulse doubler utilizing self-induced transparency

ABSTRACT

Two circularly polarized components, displaced in both time and space, of a polarized light pulse are obtained in an optically resonant medium when the medium operates under the conditions of self-induced transparency. By operating the resonant medium near, but not at, exact resonance and therefor at a position slightly displaced from the absorption resonance line of the medium, application of an axial magnetic field causes an impinging polarized light pulse to be separated into two circularly polarized components.

350-315 5R f SEARCH ROOM United State! Courtens Mar. 21, 1972 OPTICALPULSE DOUBLER UTILIZING References Cited SELF JNDUCED TRANSPARENCY OTHERPUBLICATIONS [72] Inventor: Eric Councils Adhswll Swmerland McCall etaL, Self-induced Transparency" Physical Review [73] Assignee:international Business Machines Corpora- 2 ly 1969) PP- 485 tion,Armonk, NY.

Primary ExaminerDavid Schonberg [22] Wed: 1970 Assistant Examiner-PaulR. Miller [21 1 AppL 63,349 I AttorneyHanifin & Jancin and FrankChadurjian [57] ABSTRACT [30] Foreign Application Priority Data I Twocircularly polarized components, displaced in both time Aug. 25, 1969Switzerland 12845/69 and space of a polarized light pulse are obtainedin an optically resonant medium when the medium operates under the 521U.S. Cl ..350/151, 350/150, 350/157 conditions of self-inducedtransparency. By operating the [51] int. Cl. ..G02f 1/22 resonant mediumnear, but not at, exact resonance and [58] Field of Search ..350/l50,I51, 154, 160; therefor at a position slightly displaced from theabsorption 331 /94.5 resonance line of the medium, application of anaxial magnetic field causes an impinging polarized light pulse to beseparated into two circularly polarized components.

14 Claims, 8 Drawing Figures 4 l l I 0 0 OH? i J\ A A J;A

BACKGROUND OF INVENTION & PRIOR ART This invention relates to a methodand apparatus for the generation of separated optical pulses making useof nonlinear optical propagation effects.

The quantum electronic effect of "self-induced transparency is anonlinear optical propagation effect whereby a normally opaque mediumbecomes transparent to coherent light pulses. The light pulses areabsorbed and continuously re-emitted by the system being excited at itsoptical resonant frequency, and the intensity and shape of the pulsesre-emitted remain essentially unchanged. In this manner, the mediumbecomes transparent to the laser light, the latter penetrating themedium without any appreciable attenuation. This effect has beenreported in the literature by, among others. S. L. Mc- Call and E. L.Hahn in a paper entitled Self-induced Trans parency by Pulsed CoherentLight," Physical Review Letters, Vol. 18, No. 2! of May 22, I967, pages908-9l I. This paper deals with a system comprising a ruby laser as anoptical transmitter and a ruby crystal as an optically resonant medium.C. K. N. Patel and R. E. Slusher have also described the effect in theirpaper entitled Self-induced Transparency in Gases," Physical ReviewLetters, Vol. 19, No. l8 of Oct. 30, I967, pages 1019- l022.

Reference is made in the literature to the so-called 21r-pulses, whichpulses are explained in terms of the Feynman vector model that relatesto the Schrodingers equation. In this model the pseudo-dipole momentvector runs through a complete circle when a two-level system is excitedfrom its ground state to the upper level and then' back to its groundstate (Journal of Applied Physics, Vol. 28, No. 1, Jan. I957, pages49-52). One of the known properties of these 21r-pulses lies in the factthat, within the resonant medium their propagation velocity, i.e., theirsignal velocity, is strongly decreased under the conditions ofself-induced transparency. This effect is utilized in the implementationof this invention and is also described by the inventor in a paperentitled Giant Faraday Rotations in Self-Induced Transparency," PhysicalReview Letters, Vol. 2i, No. l of July l, I968, wherein it is statedthat with the resonance effect of self-induced transparency there isconnected not only a strongly decreased propagation velocity of light,i.e., the signal velocity, but also a giant Faraday effect. This noveleffect is also described by the inventor in a copending applicationentitled Light Modulation by Resonant Faraday Effect," U.S. Pat. No.3,602,574, issued on Aug. 31, I97 I which is assigned to the assignee ofthis application.

For further background, reference is made to an article by S. L. McCalland E. L. Hahn appearing in the Bulletin American Physical Society, Vol.18, No. 21 (I967) at page I I89 entitled, Coherent Light PropagationThrough an Inhomogeneously Broadened Two-Level System" and an article byC. K. Rhodes, A Szoke and A. Javan entitled, The Influence of LevelDegeneracy on the Self-Induced Transparency Effect," Physical ReviewLetters, Vol. 21, No. l6 of Oct. 14, 1968, pages I l5l ll55.

SUMMARY OF THE INVENTION In operation of devices of the typedisclosed inthe aforementioned ccpending application, where a train of pulses from amode locked laser was utilized for data transmission, it was found thata natural upper-limit exists regarding the achievable separation ofgenerated light pulses with respect to time and space. This separationis usually limited to the cavity round-trip time which is defined by theratio of twice the.

center frequency of the optical pulses is chosen such that the resonantmedium operates near, but not at, exact resonance and therefor at aposition slightly displaced from the absorption resonance line of themedium. An axial magnetic field is then applied to the medium and actsin'sucha manner that each of the impinging optical pulses are separatedin time and space into two circularly polarized components.

Accordingly, it is an object of this invention to provide an improvedmethod and apparatus for the modulation of light.

It is a further object of this invention to provide an improved lightgenerator.

A further object of this invention is to provide a light pulse generatorwhich is capable of attaining a self-induced transparency state which isoperated at a position displaced from its absorption line to provide twocircularly polarized components ofeach impinging light pulse.

Another object of this invention is to provide a light pulse generatorwhich is energized by a polarized light pulse source to attain aself-induced transparency state when resonating at a point near, but notat, exact resonance, wherein application of an axial magnetic fieldcauses the impinging optical pulses to be separated in time and spaceinto two circularly polarized components.

Still a further object of this invention is to provide a method ofgenerating closely separated optical pulses by passing BRIEF DESCRIPTIONOF THE DRAWINGS FIG. I illustrates a spectral distribution function ofthe resonant centers, of arbitrary shape, in the case of absence of amagnetic field, and its separation into two similar functions displacedfrom each other by the Zeeman-splitting in case of presence of an axialmagnetic field.

FIG. 2 schematically illustrates the relation of the optical frequenciesof the light pulse and of the absorption line ac cording to thisinvention.

FIG. 3 schematically illustrates an application of the method fordoubling a train of optical pulses whereby means are provided to changethe state of polarization of the pulse train and to maintain the properconditions for self-induced transparency.

FIG. 4 illustrates a means by which given pulse sequences may bemultiplied.

FIG. 5 is a schematic block diagram of an embodiment of this inventionwhereby linearly polarized laser pulses are separated by a magneticfield of modulating microwave signals with respect to time and spaceinto pulse pairs consisting of the two circularly polarized componentsof the original optical pulses.

FIG. 6 illustrates the propagation velocity of the envelope of theoptical pulses for the two circularly polarized components as a functionof the axial magnetic field acting ,upon the resonant centers inconjunction with a modulating magnetic field having a phase velocityV,,,. g

FIGS. 7a and 7b illustrate a possible coding scheme.

In FIG. I a solid line represents an arbitrary spectral distributionfunction g(Aw) which describes the absorption of radiation by anexcitable resonant center. With its aid the properties of an opticallyresonant medium can be described quantitatively. Said distributionfunction is valid in the case of absence of the magnetic field.

In case an axial magnetic field is acting. there occurs a magnetic levelsplitting according to the Zeeman-effect. The spectral line separatesinto two or several Zeeman lines. For explanation purposes a Kramer'sdoublet is taken as the twolevel system, i.e., a transition from a statewith the quantum number .i one-half to another state with the quantumnumber .l one-half. ln this case, upon the application of a magneticfield parallel to the direction of propagation z of the optical field,the spectral line splits into two components. This is illustrated in H6.1 by the two distribution functions, represented by dotted lines,g,,(Au) g(Aw-w,) and g (Aw) g(Am+w,), which are separated by theZeemansplitting (0,. Here, the independent variable Aw is defined asdifference Aw =mQ of the running frequency w and the frequency oftheoptical carrier. This difference, i.e., the coordinate A0) is posi' tivewhen the independent variable to is greater than the frequency of theoptical carrier. All frequencies are measured in angular frequency. Theselection rule is such that only AM =21 transitions take place. The'g(Aw) line corresponds to one sense of circular polarization (AM =+l andthe other line to the other sense of polarization.

Some definitions are now given ofquantities which are used in the abovecited literature sources in a similar way. A quantity characteristic ofthe duration of the optical pulses is denoted by 1-. lt essentiallycorresponds to the halfwidth, i.e., the pulse duration or timeseparation between the wings ofthe pulse measured at half intensity.This quantity 1 must be shorter than the homogeneous part of thetransverse relaxation time of the resonant transition. This assumptionis necessary so that the influence of relaxation effects between thelevels can be neglected.

For simplification it is also assumed that k7 Iiw,, i.e., the thermalenergy is much larger than the Zeeman splitting energy (k is theBoltzmann-constant, T is the absolute temperature,? is the Planck'sconstant divided by 211, and w is the Zeeman-splitting measured asangular frequency). Thus, it is assumed that the sub-levels of theground state are equally populated. This is by no means necessary, butsimplifies the following description.

The doubling ofan optical pulse is achieved in the following manner. Alinearly polarized pulse is impinging upon the resonant medium in such amanner that transparency occurs. The two circularly polarized componentsof this pulse propagate with different velocities due to the action ofan applied magnetic field, as explained below.

The spectral distribution function g(Aw) is valid for the unsplitspectral line in the absence ofa magnetic field. Due to the action of anapplied axial magnetic field both spectral distribution functionsbelonging to the components of the split pulse g(Aw (0,) are separatedfrom each other by the Zeeman-splitting. The propagation velocities ofeach component can be determined from the given quantities. The inversepropagation velocity is given by w s: L

Here, n designates the refractive index of the optically resonant mediumwithout taking into consideration the contribution of the resonantcenters, i.e., the refractive index of the mere host lattice in case ofa crystal medium, and c the vacuum velocity of light, i.e., thepropagation velocity of optical energy in empty space.

The system constant A 41r Nw p /nch combines some properties of thetwo-level system used and several natural constants. N represents thedensity of resonant centers in one of the Zeeman sub-levels interactingwith the electromagnetic radiation; p designates the strength ofinteraction, by making it equal, e.g., to the xor y-component of themacroscopic dipole moment of the transition. The influence of themagnetic field is taken into account by the magneto-gyric ratio 7w,/H=gB/li. the ratio of Zeeman-splitting to the field strength H of themagnetic field; g is the so-called g-factor of spectroscopic separationfactor which is also called Lande-factor; and [3 is the Bohr-magneton.

The quantum electronic effect of self-induced transparency is an opticalresonance effect whereby a normally absorbing medium becomesnon-absorbing to coherent light pulses having an electric field with adetermined strength. The two-level system excited with its opticalresonance frequency continuously absorbcs and re-emits the impinginglight pulse, thereby intensity and pulse shape remain substantiallyunchanged. Thus, the medium becomes so to say, transparent, to the laserlight which passes practically without loss. After a vector modelintroduced by Feynman, Vernon and Hellwarth, which model describes thesolution of the special Schrodinger equation for a two-level system,said vector runs through a complete circle or 27l' when the system isexcited from ground state to an upper level and returns to its lowerlevel completely and in proper phase relationship. Light pulsesfulfilling exactly this condition under the considered system are,therefore, called 21r-pulses in the literature. The time integratedfield strength of the impinging coherent and linearly polarized opticalpulses must fulfill the 21r condition; furthermore, the pulse shape,i.e., the envelope of the optical carrier, is also determined. Furtherdetails may be taken from the above-cited literature sources.

To carry out the method for the generation of closely separated opticalpulses. such 21r-pulses are impinging upon an optically resonant mediumchosen as suitable two-level system. However, operation is not effectedat sharp resonance, i.e., at exactly matching frequencies of the opticalcarrier and of the absorption line, but at a point displaced from exactresonance by a certain amount. If. in addition, an axial magnetic fieldis acting upon the resonant centers, within a certain effective lengthof resonant medium, then the originally linearly polarized optical pulseseparates into its two circularly polarized components which areseparated in this manner from each other with respect to time and space.

This operation mode is not to be identified or to be confused with amodulation method for 21r-pulses thereby with the application of anaxial magnetic field a strong rotation of the plane of polarization ofthe optical pulses is obtained. By this latter method, operation isperformed at exact resonance. The Faraday rotation is caused by thedifferent dispersion characteristics of the two Zeeman sob-levels, andthis is maximum at exact resonance.

Contrary to said modulation method, when carrying out the method of thisinvention the different absorption properties" of the medium withrespect to the two circularly polarized components of the optical pulsesare used, creating a difference between the propagation velocities ofthe components. Absorption is put between quotation marks because onecannot speak of absorption in the usual sense when speaking of theeffect of self-induced transparency. At exact resonance the applicationof a magnetic field does not cause a L is the length of the resonantmedium required to achieve separation of the pulse components by adistance 27 at the output of the separating device by the application ofa unit field strength of magnetic field H. A positive L value shows thatwith positive magnetic field H the o-,component is the first to arriveat the output.

FIG. 2 schematically shows the relation of the optical frequencies oflight pulse and absorption line when carrying out the inventive method.Operation is done in the offresonance state. The optical pulse with thecarrier frequency (1 is displaced from the center of the absorption lineby the amount 6 measured as angular frequency. The center frequency ofthe optical pulses is chosen to be equal to a frequency on a wing of theabsorption line. Only in this way the separation in time and space ofthe circularly polarized components of the optical pulses, when they areabsorbed at resonance and re-emitted, can be achieved. The above definedvalue L' of the inverse separation length becomes zero in case ofresonance. That means that the inventive method is only feasible whenthe center frequencies of the pulses and of the absorption line aredisplaced from each other by a certain amount.

Without being true to scale, the dotted lines of FIG. 2 schematicallyillustrate the envelope of an optical pulse with its center frequency 0,while the solid lines illustrate the distribution function g(w) of asharp absorption line the center frequency of which is displaced fromthe frequency by the amount 6.

To estimate the values of the parameters for the inventive method, theborderline cases of a sharp absorption line and of a broad absorptionline are considered.

' For a sharp absorption line, a profile g(Am) 8 (Am 5) is taken, where6 is a Dirac delta function. The calculation ofthe inverse separationlength gives in this case ama The optimum value of the frequencydisplacement pulse duration product is found to be 5r=l/\ i.e., this isthe value which causes a maximumsepa r ation of the two pulse componentsfor a given y and unit field. Inserting this value leads to the maximuminverse separation length, i.e., the minimum required length forseparation (L") max 0.65 A-yr".

For a broad absorption line a Gaussian distribution is considered:

T T(Aou5) 7.. l--2l Here. Tis a parameter of the distribution andcorresponds to approximately the half-width of the absorption line atmaximum intensity. The calculation of the inverse separation lengthleads to the following integral:

for the *s,, P,,, transition in potassium vapor at 300 C. A7

=7,3.1o' Oecm."sec.". For the A, tar-minim transition in ruby with 0.05%Cr*concentration at liquid helium temperature, this value is A7 6.10"Oe"cm.' secf'.

of the impinging optical pulses. The line width 1' consists of twoportions, the first portion 1" being the contribution of the homogeneousline broadening, and the second portion I being the contribution of theinhomogeneous line broadening. The line width is measured in angularfrequency as half the width at half-intensity. The reciprocal T, III" iscalled the transverse relaxation time, that is, the meantime in whichthe coherence between virtual spins is destroyed by statisticalprocesses of the kind of collision processes or by other influencesdisturbing the phase relation. in a similar manner this relaxation timeconsists of two portions T 1/1", and T5 1/1". The homogeneous relaxationtime T of the resonant transition must be larger than the pulse duration7. Many combinations of light sources and materials are possible whichmay be used with the described method. I

Normally, solid state materials that are to be excited to selfinducedtransparency by the giant pulses of usual Q-switched giant pulse lasersmust be cooled to ensure that the homogeneous relaxation time is longerthan the duration of the used giant pulses. When Q-switching a giantpulse laser, optical pulses can be generated which have a pulse durationin the order of magnitude of 10 through 10 seconds. Therefore, the rubycrystal in the slow wave structure of the below described system rubylaser-ruby crystal has to be cooled down to helium temperature. However,if as an optical transmitter a modelocked laser is used, pulse durationsin the order of magnitude of 10 seconds are feasible. Since many solidstate materials show transverse relaxation times longer than 10'seconds, systems which can operate at higher temperatures, even at roomtemperature, are also possible.

An increasing number of so-called coincidences are becoming knownbetween laser lines and excitable electronic or vibrational transitionswhich can be used for self-induced transparency and hence for thedescribed method. These possibilities multiply by the application oflaser structures tunable over a wider spectral range or by theapplication of parametric conversion for the generation of coherentlight. Thus, the radiation of the optical transmitter can be tuned andmatched to the requirements of the absorber material.

in the following, a special system ruby laser ruby crystal is used forexplanation purposes in which the laser operates at nitrogen temperatureand the ruby crystal at helium temperature. The cooling of the rubyabsorber at liquid helium temperature ensures a sufficiently largehomogeneous relaxation time. The cooling of the laser at liquid nitrogentemperature brings the output frequency due to the transition A, (t 3/2E (E) in coincidence with the absorption frequency of the ruby crystalat helium temperature due to the transition A 1/2) E (E).

The parameters of the system being considered, i.e., of the rubycrystal, take the following values. The effective number of excitationcenters per volume, i.e., of the Cr *ions can be N 10'' cm whichcorresponds to 0.05% Cr cOncentr ation. The parameter characteristic ofthe strength of interaction betweenn field and matter, i.e., the squareof the dipole moment of the transition, is p 5' 10"" erg cm"; therefractive index of the host crystal Alp, is n 1.76. The angularfrequency of light is approximately at 2.7 l0sec The gyromagnetic ratiois y 2 l0emu. Hence, the product characteristic of the properties of thetwo-level system becomes A7 6 l0" 0e sec 2, where the system constant Ais defined by'A 41'rN wp n h, H

At the chosen low temperature the inhomogeneous absorption line shows aprofile with Gaussian distribution having a linewidth at half intensityof about 0.1 cm. The characteristic time of this distribution, evaluatedfrom this linewidth,

is T 1.2 Isec. The off-resonance operation poigt should be at the lowerfrequency wing of the A (i /Q)++ E(E) transition. The next followinghigher frequency A; (:3/2) E (E) transition is displaced from the linemaximum by about 0.38 cm". lts influence can be disregarded since themagnetic fields to be considered with the inventive method are lowerthan I Kilogauss.

It is now assumed that the optical pulse duration is longer than thecharacteristic time describing the linewidth ol'the active medium. Thefollowing evaluation must be based on the estimate which applies to abroadened absorption line; as given above. In this case the inverseseparation length can be found by evaluating the following integral:

The meaning of all magnitudes used in the formula is the same as abovementioned. The time parameter of the distribution is designated T, thepulse duration 1', and 5 has the meaning of the displacement of theoperation point as illustrated by FIG. 2. For simplicity reasons, anauxiliary quantity or is defined by E 017. With the introduction of theauxiliary quantity or and with the substitution uT/r=x, the integral canbe transformed and simplified in the following manner:

The denominator of the integrand is minimum and equal to unity when x01. Since at is assumed to be much larger than T, the main contributionto the integral occurs in the region x= a. In that region the numeratoris approximately constant and equal to 0: exp [-oF/Z]. This numeratorhas its maximum value for a 1. Hence, a l is the value ofa for which theintegral is maximum. According to the above definition of the auxiliaryquantity or, this means that the optimum operation point for the methodof generation of closely separated optical pulses lies at a distanceE=l/T offlresonance.

To estimate the integral the numerator of the integrand can beconsidered constant and equal to exp /](in case :1 =1). This leads tothe optimum inverse separation length: L hr/2e A y T 0.66 cm. Therefrom,the optimum value of active length of the optically resonant materialcan be calculated to be I50 cm. for a magnetic field of l Gauss. Ofcourse, the active length of the actual device can be smaller if one canapply higher magnetic fields, or ifone is satisfied with a smaller pulseseparation than 21.

Two distinct embodiments that utilize the system ruby laser ruby crystalfor the purpose of pulse doubling will now be explained.

First, a train of impinging optical pulses is considered that may have aduration of about I nsec and are separated in time by ID nsec. Then 1'I0 sec and the pulse repetition rate is 10 per sec. This pulse sequenceis to be doubled and quadrupled, in two steps into a sequence ofnsec-pulses following each other at a distance of 2.5 nsec. FIGS. 3 and4 schematically indicate this method. A train of linearly polarizedpulses impinges upon a first pulse doubler device I. The polarizationstate is indicated by respective arrows above the symbols for thepulses. The output signals are a train of light pulses with half thedistance compared to the impinging pulses. These output pulses arealternately left-circularly and right-circularly polarized, assymbolically indicated by the curved arrows.

Device I consists of a ruby crystal cooled to liquid helium temperaturefor the reasons explained above, and placed in the axis of a solenoidcapable of producing an axial magnetic field of a few hundred Gauss.From the value L,,,,,,= I50 cm.

that has been calculated above, it is seen that a field strength ofabout 300 Gauss applied to a ruby crystal of about l.3 cm. length issufficient in the present example to cause a separation ofthe opticalpulses by 5 nsec.

For further processing the pulse sequence the train of circularlypolarized pulses which is thus produced is passed through a quarter-waveplate to change it into a train of linearly polarized pulses. The outputpulse train emerging from the M4 plate has been changed, however, to asequence of pulses alternately polarized in two orthogonal directions,in view of the history of said pulses. This fact is indicated in FIG. 3by the symbolic arrows showing a sequence of light pulses beingpolarized alternately vertically and horizontally. To fulfill everywherethe 21'r-condition, i.e., to have the appropriate intensity forself-induced transparency, and to compensate for diffraction losses. theoptical pulse sequence is to pass a lens system. This is schematicallyshown at the right-hand side of FIG. 3. In this way a train of linearlypolarized light pulses with doubled pulse repetition rate is obtained.

As shown in FIG. 4 this pulse sequence, after having been doubled withthe aid ofthe first device, is now impinging upon a second such pulsedoubling device ll. Hence, the output pulse train consists ofalternately left-circularly and right-circularly polarized opticalpulses following each other at a quarter of the distance of the originalpulse sequence. Again, a quarter-wave plate provides for the change ofthe circularly polarized optical pulses into a train oflinearlypolarized pulses which may be further processed, accordingly. For thesame crystal length as in the doubling device I, this second devicewould do with a field of half the strength, i.e., of approximately I50Gauss.

. As a further embodiment, a pulse train should be modulated with binaryinformation. FIGS. 5, 6 and 7 are used for explanation purposes. Asoriginalpulsesequenee the same train of nsec pulses is taken. as shownabove, following each other at a distance of 10 nsec. Hence, the pulseduration is again 1 IO sec., and the pulse repetition rate is 10 sec..

The modulation device corresponds to the scheme shown in FIG. 5. Thesame material, i.e., pink ruby with about 0.05%

Cr ion concentration, is used under the same conditions as above. Hence,all parameters of the two-level system are the same. The operationpoint, i.e., 5, is also chosen in the same way.

Here the axial magnetic field for separating the optical pulses intotheir components is generated by a microwave signal. The phase of thissignal is fixed with respect to the pulse separation of the impingingpulse train. A master oscillator 10 provides this time coincidence ofthe microwave signals generated by the microwave pulse generator l2 andof coherent and polarized light pulses generated by the opticaltransmitter 14. The optical transmitter I4 is a mode-locked ruby laserwith a light modulator I6 in the cavity. In other application theoptical transmitter could be any kind of transmitter producing polarizedlight pulses of an optical frequency which, under consideration of therequirements of the inventive method, lies within the range of theresonant frequency of the medium used for self-induced transparency. Itmay be, for instance, a solid state laser or a gas laser of suitableintensity. For the generation of so-called giant pulses a Q-switch ofthe laser cavity may be used. Also, a mode-locked laser may be used, orany other method for the generation of short and intensive opticalpulses which utilizes parametric processes.

in the optical transmitter 14 the light modulator 16 controls therepetition rate of the generated optical pulses, according to thefrequency of the master oscillator 10. Since this light modulator 4 isswitched only by pure periodical signals, a Pockels cell may,forinstance, be used.

In the FIG. 5, the optical transmitter I4 is schematically shown as asolid state laser. On the right-hand side, the rectangle including anarrow indicates a polarizer which may be replaced by other polarizingmeans, eg. a laser provided with Brewster-windows does generate linearlypolarized coherent light pulses without any other polarizing means.

In the optical axis of laser 14, there is a microwave slow wavestructure 18 which contains the resonant material upon which the lightpulses impinge thereby causing the effect of self-induced transparency.The slow wave structure is designed so as to produce an axial componentof the microwave field. The phase velocity of this microwave field is,in the delay device 18, made equal to the propagation velocity ofthelight pulses.

The linearly polarized light pulses generated by the optical transmitter14 follow each other equidistantly and impinge upon the slow wavestructure 18 where they are strongly delayed by the optically resonantmedium. The axial component of the magnetic field of the microwavepulses running concurrently with the light pulses causes an increasingseparation of the optical pulses into their components.

in the drawing there is symbolically indicated by respective arrows thatthe impinging optical pulses are linearly polarized and that the pulsepairs emerging from the device l8 are polarized right-circularly orleft-circularly, respectively. In the first case the vertical arrowsindicate the linear polarization state, in the second case the curvedarrows indicate the respective circular polarization state. The delay ofthe propagation of the optical pulses within the slow wave structure 18is indicated by the shorter distance of the centers of the pulse pairsfollowing each other, when being compared to the distance of theimpinging pulses.

Modulation of the optical pulses with an information is possible bycontrolling the axial magnetic field, i.e., by controlling the microwavepulse generator. For a binary notation both binary values may berepresented by doubled pulses or single pulses, respectively. Thispossibility is indicated in the drawing by control input 20 of microwavepulse generator 12. Suitable control signals can be supplied to thatinput, i.e., from a data processing system. At signal output 22 ofmicrowave pulse generator 12 modulated microwave signals are thenavailable which can modulate the impinging optical pulses within theslow wave structure 18 via the excited resonant centers ofthe activemedium used.

For instance, the slow wave structure 18 may consist of a delay line formicrowaves which is built ofa helix surrounding a core of the opticallyactive material chosen. The phase velocity of the microwave is smallerthan the vacuum light velocity within the slow wave structure. It ismatched to the propagation velocity of the optical pulses which isstrongly reduced in the optical resonant medium when self-inducedtransparency occurs. While both kinds of electromagnetic waves aretraveling together, the optical pulses are separated into theircircularly polarized components by the action of the magnetic field ofthe microwave upon the resonant centers. The impinging optical pulsesare directed axially upon the core, and the helix is fed with themicrowave signal.

The required magnetic field strength is now delivered by the modulatingfield of microwaves. For the representation of one binary value thecorresponding light pulses need only be identifiably doubled, or solelybroadened, and, therefore, one can do with relatively small fields.There are two kinds of electromagnetic waves propagating in themodulator. the modulating microwave and the optical pulse. These wavesare in relative motion, and it can be seen that there exist equilibriumpoints where both kinds of waves run at the same velocity. This causes asaturation of the achievable pulse separation. This effect can beexplained by the relative motion of the optical pulses separating intotheir circularly polarized components with respect to the modulatingmicrowave field, or

better to say, to the longitudinal magnetic component H in Z- direction.Thus, the optical pulses are in regions of varying local field strength.This motion, relative to the running field, stops when the opticalpulses reaches a field strength value in an equilibrium point where itspropagation velocity is equal to the phase velocity of the field.

As shown in FIG. 6, points where the pulse velocity is equal to themicrowave phase velocity do exist; they can be either stable or unstablepoints. In the upper part of FIG. 6 a solid line shows the propagationvelocity V of the right-circularly polarized component of the opticalpulses as a function of the magnetic field strength H. It is designatedR. In a similar manner, a dashed line designated L shows the propagationvelocity of the left-circularly polarized component of the opticalpulses. it can be seen that at a certain positive value of the magneticfield strength the right-circularly polarized component reaches theminimum propagation velocity V,,,,,,, and that at a correspondingnegative field strength value the leftcircularly polarized component ofthe optical pulses reaches the minimum value of the propagationvelocity. The upper asymptotic boundary limit of the maximum propagationvelocity is given by the refractive index of the host lattice and hasthe value c/n. Between both thin lines representing the boundary values,another thin line shows a propagation velocity value designated V,,,,,which is the value of the phase velocity of the microwave in the slowwave structure. This line intercepts the lines R and l, at field valuesH," which correspond to either stable or unstable equilibrium points.

The lower part of FIG. 6 shows at one instant of time the periodicallyvarying component of H, running in the Z- direction with phase velocityV,,,,. Dashed auxiliary lines indicate the positive and negative value Hof the magnetic field strength at which the pulse velocity is equal tothe microwave velocity. These auxiliary lines intercept the curverepresenting H, at various spatial points (24, 26, 28) and it will nowbe shown that half of these are stable propagation points and the otherhalf unstable points.

it is assumed that a right circularly polarized pulse has reached point24; the value of the magnetic field strength is H,,,, and thepropagation velocity is the same as the phase velocity. if the opticalpulse accidentally moves faster than the microwave field, it gets into aregion of smaller field strength, as can be seen from the lower part ofthe figure. However, from the upper part of the figure it is seen thatat the same time its propagation velocity decreases. As a result theoptical pulse slows down immediately. Similarly, if the pulse triesslowing down with respect to the microwave, it comes into an increasingfield which accelerates it. Therefore, 24 is a stable point ofdynamicequilibrium.

Point 26 shown by a cross is an unstable equilibrium point fortheright-circularly polarized optical pulse. If the optical pulseaccidentally moves faster than the microwave it comes into an increasingand accelerating field. lt surpasses the maximum till with decreasingfield it reaches the propagation velocity V,,,, corresponding to thestable point 24. Left-circularly polarized pulses behave analogously.Equilibrium points shown as thick points in the figure are stable, thoseshown by a cross are unstable.

This analysis shows that the maximum achievable pulse separation in thistype of modulator is equal to one half of the microwave period. Forinstance, a linearly polarized pulse at a spatial position 2 betweenpoints 24 and 26 in the lower part of FIG. 6 will progressively separateinto its two circularly polarized components, until these reach theirpositions 24 and 28, and from there on will keep this constantseparation. This is very advantageous from a coding point of view, andallows for theunambiguous transmission of digital information,independently of the exact length of resonant material and of the exactstrength of the magnetic field, provided these are above some minimumvalue.

A 0 ruby crystal rod with a diameter of the order of one millimeterserves as a core ofthe helix. The ratio of conductor length to the helixlength is about 30. The desired delay of the electromagnetic waves is inthe order of magnitude of for this helix ratio when due account is takenof the dielectric constant of ruby at microwave frequency which is about9.5 for the TE-wave. To supply a suitable axial magnetic field componentthe microwave power of the lowest order H-mode must be of the order ofafew watts if the diameter of the ruby rod is chosen small enough, e.g.,smaller than about 1 millimeter. The laser beam is focused in the rubycrystal. The length of the crystal core is chosen to be of the order ofa few cenl timeters and depends on what separation of the pulsecomponents is desired and what diffraction losses can be tolerated.

Coding can be done in several ways. For instance, the binary informationcan affect the phase of the microwave so that each linearly polarizedpulse is doubled. but the time sequence R-L or L-R carries theinformation. Another method of coding is by the presence or absence offield. In this latter case, the pulses are either doubled or notdoubled. FIG. 7 shows an example of coding by this method. A train oflinearly polarized light pulses according to FIG. 7a is coded accordingto the binary information in such a manner that according to FIG. 78 outof the four shown pulses, both pulses in the middle are doubled intopulse pairs, consisting of their circularly polarized components. Themodulating microwave may have a frequency of 250MHz, and the pulse trainto be modulated consists of nsec pulses following each other at adistance of 10 nsec. In view of the peculiarity of the equilibriumpoints elucidated above, the pulse separation amounts to half theperiod, i.e.. to 2 nsec. For a modulating structure having a length ofabout l5 cm. the magnetic field strength necessary for a separation of2nsec. is ofthe order of H) Gauss.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

lclaim:

1. An optical pulse doubler comprising:

an optically resonant medium capable of attaining a self-inducedtransparency state at resonance;

means for applying polarized light pulses to said resonant medium tocause said medium to attain the self-induced transparency state and toresonate near, but not at, exact resonance and therefor operate at aposition displaced from the absorption line of said medium; and

means for applying a magnetic field to said medium which is parallel tothe propagation of light in said medium for separating impinging lightpulses into two circulary polarized components displaced both in timeand space.

2. The pulse doubler of claim 1, including means for selectivelycontrolling both the magnitude and direction of magnetic field appliedto said medium.

3. The pulse doubler of claim 2, including means for trans lating saidcircularly polarized pulses into a sequence of linearly polarizedpulses.

4. The pulse doubler of claim 3, wherein said optically resonant mediumis ruby.

5. The pulse doubler of claim 4 wherein said means for applyingpolarized light pulses includes a mode-locked ruby laser.

6. The pulse doubler of claim 5 wherein said means for applying themagnetic field includes a microwave pulse generator.

7. An optical pulse doubler comprising: an optically resonant mediumcapable of attaining a self-induced transparency state at resonance;means for applying coherent polarized light pulses to said resonantmedium having a duration less than the relaxation time of said resonantmedium and effective to cause said medium to attain the self-inducedtransparency state and to resonate near, but not at, exact resonance andtherefor cause operation of said medium slightly displaced from itsabsorption line;

means for selectively applying a predetermined magnetic Field to saidmedium parallel to the propagation of light therethrough and of amagnitude which is dependent upon the length of said medium and thewidth of the impinging pulses for selectively separating impinging lightpulses into two circularly polarized components displaced both in timeand space.

8. The pulse doubler of claim 7 wherein the magnetic field intensity isselectively controlled in both magnitude and direction.

9. The pulse doubler of claim 8, including information input means.connected to said means for applying said magnetic field, operable tocontrol the field applied to said resonant medium. 1 I

10. The pulse doubler of claim 9, wherein said information input meansincludes a microwave pulse generator.

11. The pulse doubler of claim 8, including information input means,connected to said means for applying said magnetic field, operable tocontrol the direction of the field applied to said resonant medium.

12. The pulse doubler of claim 11, including means for translating saidcircularly polarized pulses into a sequence of linearly polarizedpulses.

13 The pulse doubler of claim 12, wherein said optically resonant mediumis ruby.

14. A method of generating two circularly polarized components,displaced both in time and space, ofa single coherent polarized lightpulse, comprising:

applying coherent polarized light pulses to an optically resonant mediumsuch that said medium resonates near, but not at, exact resonance andattains a self-induced transparency state, and

applying an axial magnetic field to said resonant medium whose magnitudeis dependent upon the length of said medium and the width of theimpinging light pulses whereby impinging polarized light pulses areseparated into two circularly polarized components displaced both intime and space.

1. An optical pulse doubler comprising: an optically resonant mediumcapable of attaining a self-induced transparency state at resonance;means for applying polarized light pulses to said resonant medium tocause said medium to attain the self-induced transparency state and toresonate near, but not at, exact resonance and therefor operate at aposition displaced from the absorption line of said medium; and meansfor applying a magnetic field to said medium which is parallel to thepropagation of light in said medium for separating impinging lightpulses into two circulary polarized components displaced both in timeand space.
 2. The pulse doubler of claim 1, including means forselectively controlling both the magnitude and direction of magneticfield applied to said medium.
 3. The pulse doubler of claim 2, includingmeans for translating said circularly polarized pulses into a sequenceof linearly polarized pulses.
 4. The pulse doubler of claim 3, whereinsaid optically resonant medium is ruby.
 5. The pulse doubler of claim 4wherein said means for applying polarized light pulses includes amode-locked ruby laser.
 6. The pulse doubler of claim 5 wherein saidmeans for applying the magnetic field includes a microwave pulsegenerator.
 7. An optical pulse doubler comprising: an optically resonantmedium capable of attaining a self-induced transparency state atresonance; means for applying coherent polarized light pulses to saidresonant medium having a duration less than the relaxation time of saidresonant medium and effective to cause said medium to attain theself-induced transparency state and to resonate near, but not at, exactresonance and therefor cause operation of said medium slightly displacedfrom its absorption line; means for selectively applying a predeterminedmagnetic field to said medium parallel to the propagation of lighttherethrough and of a magnitude which is dependent upon the length ofsaid medium and the width of the impinging pulses for selectivelyseparating impinging light pulses into two circularly polarizedcomponents displaced both in time and space.
 8. The pulse doubler ofclaim 7 wherein the magnetic field intensity is selectively controlledin both magnitude and direction.
 9. The pulse doubler of claim 8,including information input means, connected to said means for applyingsaid magnetic field, operable to control the field applied to saidresonant medium.
 10. The pulse doubler of claim 9, wherein saidinformation input means includes a microwave pulse generator.
 11. Thepulse doubler of claim 8, including information input means, connectedto said means for applying sAid magnetic field, operable to control thedirection of the field applied to said resonant medium.
 12. The pulsedoubler of claim 11, including means for translating said circularlypolarized pulses into a sequence of linearly polarized pulses.
 13. Thepulse doubler of claim 12, wherein said optically resonant medium isruby.
 14. A method of generating two circularly polarized components,displaced both in time and space, of a single coherent polarized lightpulse, comprising: applying coherent polarized light pulses to anoptically resonant medium such that said medium resonates near, but notat, exact resonance and attains a self-induced transparency state, andapplying an axial magnetic field to said resonant medium whose magnitudeis dependent upon the length of said medium and the width of theimpinging light pulses whereby impinging polarized light pulses areseparated into two circularly polarized components displaced both intime and space.